1. Field of the Invention
This invention relates to an image signal processing method and apparatus which limit the amount of toner adhesion, and more particularly to an apparatus effectively used in a color image forming apparatus, such as a color copying machine or a color printer.
2. Description of the Related Art
In the image signal processing section of a color image forming apparatus, there is provided a toner limiting section to limit the total amount of toner adhering to copying paper. The purpose of limiting toner is to prevent toner from being detached from the paper and unnecessary toner from remaining on the paper when color printing is done on a color printer.
If the maximum value of the sum total of the image signal values of cyan (C), magenta (M), yellow (Y), and black (B) is 400%, the sum total is normally limited to about 250%.
To achieve this limiting value, the sum total of the CMYK image signal values is compared with a threshold value. Then, when the sum total of the CMYK image signal values has exceeded the threshold value, the preferred values of the individual color signals and black signal are adjusted so as to limit the sum total of the CMYK image signals.
However, the above limiting method has the following problem: if an image includes pixel values equal to or larger than the threshold value, a rapid suppression (change) of gradation takes place, degrading the image quality. The technique for limiting the amount of toner adhesion has been disclosed in Jpn. Pat. Appln. KOKAI Publication 2002-103689.
In the technique disclosed in the patent document, the upper limit and lower limit which limit the image signal values are set. An image signal which exceeds the upper limit is forcibly converted into the upper limit. An image signal which is smaller than the lower limit is forcibly converted into the lower limit. Therefore, this technique has a problem: in an image which includes pixel values equal to or larger than the threshold value as described above, a rapid suppression (change) of gradation takes place, resulting in degradation of the image quality.